The document provides an overview of bipolar junction transistors (BJT). It begins with a brief history, describing how the transistor was invented in 1947 at Bell Labs by Bardeen, Brattain, and Shockley. It then explains the basic structure and operation of NPN and PNP BJTs, including the emitter, base, and collector terminals and how current flows. Finally, it discusses the different modes of operation - cutoff, saturation, and active - and how BJTs can be used as switches or amplifiers depending on the biasing conditions.

3. R.M.K COLLEGE OF ENGINEERING AND
TECHNOLOGY
DEPARTMENT OF ECE
EC8252-ELECTRONIC DEVICES
SECOND SEMESTER-I YEAR- (2020-2024 BATCH)
Mrs.P.Sivalakshmi AP/ECE
SESSION:11
DATE: 3.05.2021

4. 2017- Regulation syllabus

5. Lecture Overview on BJT
• What is a Transistor?
• History
• Types
• Characteristics
• Applications

6. What is a Transistor?
• Semiconductors: ability to change from
conductor to insulator
• Can either allow current or prohibit current to
flow
• Useful as a switch, but also as an amplifier
• Essential part of many technological advances

7. A Brief History
• Guglielmo Marconi invents radio in 1895
• Problem: For long distance travel, signal must be amplified
• Lee De Forest improves on Fleming’s original vacuum tube to
amplify signals
• Made use of third electrode
• Too bulky for most applications

8. The Transistor is Born
• Bell Labs (1947): Bardeen,
Brattain, and Shockley
• Originally made of germanium
• Current transistors made of
doped silicon

9. How Transistors Work
• Doping: adding small amounts of other
elements to create additional protons or
electrons
• P-Type: dopants lack a fourth valence electron
(Boron, Aluminum)
• N-Type: dopants have an additional (5th)
valence electron (Phosphorus, Arsenic)
• Importance: Current only flows from P to N

10. Diodes and Bias
• Diode: simple P-N junction.
• Forward Bias: allows current to
flow from P to N.
• Reverse Bias: no current allowed
to flow from N to P.
• Breakdown Voltage: sufficient N
to P voltage of a Zener Diode will
allow for current to flow in this
direction.

11. The Invention
 Important Features (compared to Vacuum tubes):
- three terminal solid-state device - requires less power
- smaller and lightweight - lower operating voltage
- has rugged construction - more efficient
- no heater requirement
The First Transistor: On Dec 23, 1947, three scientists led by Dr. William Shockley at the
Bell Telephone Laboratories demonstrated the amplifying action of the first transistor.
(Courtesy Bell Telephone Laboratories.)
Co-inventors:
Dr. William Shockley
(seated);
Dr. John Bardeen (left);
Dr. Walter H. Brattain.
Honored with
Nobel Prize in
Physics in 1956

12. Bipolar Junction transistor
Holes and electrons
determine device characteristics
Three terminal device
Control of two terminal currents
Amplification and switching through 3rd contact
The term transistor was coined by John R. Pierce as a contraction of
the term trans resistance.

13. The Structure
 Bipolar: both electrons and holes are involved in current flow.
 Junction: has two p-n junctions.
 Transistor: Transfer + Resistor.
 It can be either n-p-n type or p-n-p type.
 Has three regions with three terminals labeled as
i. Emitter (E)
ii. Base (B) and
iii. Collector (C)
The Bipolar Junction Transistor (BJT)

14. The Structure: npn & pnp
 Base is made much narrow.
 Emitter is heavily doped (p+, n+).
 Base is lightly doped (p-, n-).
 Collector is lightly doped (p, n).

15. The Structure: npn & pnp
 Transistors can be constructed as two diodes that are
connected together.

16. Circuit Symbol
 The arrow indicates the direction
of current flow.
 The current flows from collector
to emitter in an n-p-n transistor.
 The arrow is drawn on the
emitter.
 The arrow always points towards
the n-type. So the emitter is n-type
and the transistor is n-p-n type.
Layout and Circuit Symbol: n-p-n Transistor

17. Circuit Symbol
 The arrow indicates the direction
of current flow.
 The current flows from emitter to
collector in an p-n-p transistor.
 The arrow points towards the n-
type.
 So the base is n-type and
transistor is p-n-p type.
Layout and Circuit Symbol: p-n-p Transistor

18. Modes of Operation
 Based on the bias voltages applied at the two p-n
junctions, transistors can operate in three modes:
1. Cut-off (both EB and CB junctions are reversed
biased)
2. Saturation (both EB and CB junctions are
forward biased)
3. Active mode (EBJ is forward biased and CBJ is
reversed biased)
 Cut-off and Saturation modes are used in switching
operation.
 Active mode is used in amplification purposes.

19. Modes of Operation
npn bipolar junction transistor
pnp bipolar junction transistor

20. Modes of Operation
 Both the junctions are
reversed biased.
 No current can flow through
either of the junctions.
 So the circuit is open.
Cut-off
VBC
+
+
-
-
VBE
Ideal model of BJT
in cut-off.

21. Modes of Operation
 Both the junctions are
forward biased.
 So the equivalent circuit can
be represented by short-circuit
between the base, emitter and
collector.
Saturation: Ideal Model
VBC
+
+
-
-
VBE
Ideal model of BJT
in saturation.

22. Active Mode Operation
EBJ:
Forward Biased
CBJ:
Reverse Biased
◦ Forward bias of EBJ injects electrons from emitter into base (Emitter current).
◦ Most electrons shoot through the base into the collector (Collector current).
◦ Some emitted electrons recombine with holes in p-type base (Base Current)

23. +
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
N P N
E
B
C
electron
Hole
Active Mode Operation

24. +
+
+
+
+
+
-
-
-
-
-
-
N P N
E C
B
VBE VCB
E-Field
Electron diffusion
Hole diffusion
Active Mode Operation

25. +
+
+
+
+
+
-
-
-
-
-
-
N P N
E-Field
E C
B
VBE VCB
Electron hole recombination
Active Mode Operation

26. Collector current
T
BE
V
v
s
C e
I
i 
• The equation above shows that the BJT is indeed a
voltage-dependent current source; thus it can be
used as an amplifier.
Electrons that diffuse across the base to the CBJ junction are swept
across the CBJ depletion to the collector because of the higher potential
applied to the collector

27. Active Mode Operation
Biasing for Active Mode
Carriers injected from forward bias junction (from the emitter labeled E)
travel through the intermediate layer (BASE, labeled B) and swept into the
COLLECTOR, labeled C by the reverse biased voltage.
EBJ: Forward Biased
CBJ: Reverse Biased

28. Conceptual Biasing Circuits
npn Transistor

29. Conceptual Biasing Circuits
pnp Transistor

30. DC Output Characteristics
IC vs VCE Characteristics of an npn Transistor
Active Breakdown
Saturation 

31. 31
Active
Breakdown
Saturation 
Cutoff
DC Output
Characteristics
IC vs VCE Characteristics of an npn Transistor

32. Bipolar Junction Transistors: Basics
+
- +
-
IE
IB
IC
IE = IB + IC ………(KCL)
VEC = VEB + VBC ……… (KVL)

33. Current Gain:
– α is the fraction of electrons that diffuse across the narrow
Base region
– 1- α is the fraction of electrons that recombine with holes in
the Base region to create base current
• The current Gain is expressed in terms of the β (beta) of
the transistor (often called hfe by manufacturers).
• β (beta) is Temperature and Voltage dependent.
• It can vary a lot among transistors (common values for
signal BJT: (20 - 200).
BJT characteristics











1
)
1
(
B
C
E
B
E
C
I
I
I
I
I
I

34. Active Mode: Terminal Currents
Current Relationships and Amplification
C
E
B
B
C
E
E
C
E
C
I
I
I
I
I
I
I
I
I
I






/
.















B
C
B
B
C
I
I
I
I
I
and
1
1
 As  is close to unity,  is very large, typically around 100.
  represents the current amplification factor from base to
collector.
 The base current is amplified by a factor of  in the collector
circuit in the Active mode.
  is called the Forward Current Gain, often written as  F.
C
C
C
B
I
I
I
I







1

36. Operation
Region
IB or VCE
Char.
BC and BE
Junctions
Mode
Cutoff IB = Very
small
Reverse &
Reverse
Open
Switch
Saturation VCE = Small Forward &
Forward
Closed
Switch
Active
Linear
VCE =
Moderate
Reverse &
Forward
Linear
Amplifier
Break-
down
VCE =
Large
Beyond
Limits
Overload
Operation region summary

37. BJT configurations
GAIN CONFIG

38. How transistor works:
https://www.youtube.com/watch?v=7ukD
KVHnac4